When you call a named block of code (a method or function), you'll wish to pass in originating values that are changed from one call / use to the next - these are commonly known as the parameters. If I'm calling a function to calculate the area of a rectangle, for example, I would pass in the width and height, and if I'm calling a function to input a number from the keyboard I'm probably going to pass in the prompt message, and a minimum and maximum acceptable value.

There are some time, though, when you may want to leave out a parameter - let it be assumed. If I want to connect to a database, "localhost" would be a fair assumption if a host name is not given, for example.

In Python, I can specify optional parameters within the function definition by giving a name and assigning them to a default value, for example: def getload(lenny, seats, factor = 1.4):
will define a function that takes 2 or 3 parameters; if only 2 are specified, then the third one will take the assumed value of 1.4.

There are also times where you may wish to call your function with an indeterminate (and varying) number of parameters, and in this case you can add a * in front of the final named parameter and it will act as a sponge, gathering all remaining positional parameters into a list, for example: def average(first, *rest):
which is a function with at least one (but as many as you like) parameters. In practise, this isn't used all that often as it's far better to pass in a single list object from the calling code than to build a list of individual objects within the function in most circumstances.

Another possibility is that you may wish to call a function that could take a very large number of potentially different parameters, and they could be specified in any order. In Python, you can use named parameters by adding ** in front of the final parameter and you'll then be able to collect a dictionary of values. The function definition may look like: def bob(fred, cat="fullofbird", *rabbits, **kx):
with named parameters being collected in a dictionary called kx. Here's a sample call: bob(44,55,66,teeth=45,legs=3, arms=4)

Sample functions, and calls, shown above are in sample programs on our web site [here] and [here].

If you want to define the ultimate, generic function that will accept anything thrown at it, you can write your definition using the following structure: def wrapped(*args, **kwargs)

And if you want to call a function passing in such generics, you can do so in a similar way: finalanswer = why(*args, **kwargs)

Both of these example lines are taken from a recent example ([here]) I wrote to demonstrate decorators - wrappers that you can apply to functions or methods in Python for purposes such as logging calls and producing code coverage maps. (written 2011-10-04)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

Y105 - Python - Functions, Modules and Packages[4724] From and Import in Python - where is the module loaded from? - (2016-11-06)[4722] Embedding more complex code into a named block - (2016-11-04)[4719] Nesting decorators - (2016-11-02)[4662] Recursion in Python - the classic example - (2016-03-07)[4645] What are callbacks? Why use them? An example in Python - (2016-02-11)[4448] What is the difference between a function and a method? - (2015-03-04)[4441] Reading command line parameters in Python - (2015-02-23)[4410] A good example of recursion - a real use in Python - (2015-02-01)[4407] Python - even named code blocks are objects - (2015-01-28)[4361] Multiple yields and no loops in a Python generator? - (2014-12-22)[4212] Python functions - an introduction to how they work - (2013-11-16)[4161] Python varables - checking existance, and call by name or by value? - (2013-08-27)[4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)[3945] vargs in Python - how to call a method with unknown number of parameters - (2012-12-06)[3931] Optional positional and named parameters in Python - (2012-11-23)[3885] Default local - a good choice by the author of Python - (2012-10-08)[3852] Static variables in Python? - (2012-08-29)[3766] Python timing - when to use a list, and when to use a generator - (2012-06-16)[3695] Functions are first class variables in Lua and Python - (2012-04-13)[3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)[3474] Python Packages - groupings of modules. An introduction - (2011-10-11)[3472] Static variables in functions - and better ways using objects - (2011-10-10)[3459] Catching the fishes first? - (2011-09-27)[3280] Passing parameters to Python functions - the options you have - (2011-05-07)[3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)[2998] Using an exception to initialise a static variable in a Python function / method - (2010-10-13)[2994] Python - some common questions answered in code examples - (2010-10-10)[2929] Passing a variable number of parameters in to a function / method - (2010-08-20)[2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)[2766] Optional and named parameters to Python functions/methods - (2010-05-15)[2718] Python - access to variables in the outer scope - (2010-04-12)[2520] Global and Enable - two misused words! - (2009-11-30)[2506] Good example of recursion in Python - analyse an RSS feed - (2009-11-18)[2481] Sample code with errors in it on our web site - (2009-10-29)[2440] Optional parameters to Python functions - (2009-10-07)[2439] Multiple returns from a function in Python - (2009-10-06)[2011] Conversion of OSI grid references to Eastings and Northings - (2009-01-28)[1879] Dynamic code - Python - (2008-11-11)[1871] Optional and named parameters in Python - (2008-11-05)[1870] What to do with a huge crop of apples - (2008-11-04)[1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)[1790] Sharing variables with functions, but keeping them local too - Python - (2008-09-09)[1784] Global - Tcl, PHP, Python - (2008-09-03)[1464] Python Script - easy examples of lots of basics - (2007-12-08)[1202] Returning multiple values from a function (Perl, PHP, Python) - (2007-05-24)[1163] A better alternative to cutting and pasting code - (2007-04-26)[1134] Function / method parameters with * and ** in Python - (2007-04-04)[959] It's the 1st, not the 1nd 1rd or 1th. - (2006-12-01)[949] Sludge off the mountain, and Python and PHP - (2006-11-27)[913] Python - A list of methods - (2006-11-03)[912] Recursion in Python - (2006-11-02)[900] Python - function v method - (2006-10-20)[821] Dynamic functions and names - Python - (2006-08-03)[775] Do not duplicate your code - (2006-06-23)[749] Cottage industry or production line data handling methods - (2006-06-07)[745] Python modules. The distribution, The Cheese Shop and the Vaults of Parnassus. - (2006-06-05)[668] Python - block insets help with documentation - (2006-04-04)[561] Python's Generator functions - (2006-01-11)[418] Difference between import and from in Python - (2005-08-18)[386] What is a callback? - (2005-07-22)[340] Code and code maintainance efficiency - (2005-06-08)[308] Call by name v call by value - (2005-05-11)[303] Lambdas in Python - (2005-05-06)[294] Python generator functions, lambdas, and iterators - (2005-04-28)[105] Distance Learning - (2004-10-31)[96] Variable Scope - (2004-10-22)

This is a page archived from The Horse's Mouth at
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